Control device



1948. J. M. WILSON 2,448,502

CONTROL DEVICE Filed Nov. 6, 1942 r Inventor JOHN M. WILSON Gttomeg Patented Aug. 31, 1948 CONTROL DEVICE John M. Wilson, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application November 6, 1942, Serial No. 464,732

13 Claims. 1

The present invention is concerned with a control device and more particularly with one employing an electronic circuit for detecting when the impedance across a gap is rectifying.

In the co-pending application of Vilynn 0. Beam, Serial No. 450,613, filed July 11, 1942, there is described a control device which is adapted to determine whether an impedance connectedacross a gap is rectifying or not and to act in one manner when the impedance is rectitying and to act in another when the gap is either open-circuited or is bridged by an impedance which is non-rectifying. In this circuit, an electronic discharge device is employed and a plurality of connections are made between a control element of this device and a source of power, one of these connections normally being employed to maintain the control element at a value at which the amplifier is non-conductive and another such connection being employed to afiect the potential of the control element when v a gap in the connection is bridged by a rectifying impedance.

It is a general object of the present invention to provide a simpler construction than that shown in the aforesaid Beam application.

One object of the present invention is to provide a control device of the general type just described in which the control electrode of the electronic discharge device is connected to only a single point of said source of power, disregarding the cathode connection.

A still further object of the present invention is to provide such a control device in which a voltage is applied to the output circuit of the amplifier, which voltage is of such magnitude that the grid potential must be closer to the potential of the anode than is the cathode potential for the control device to pass current so thatrno special provision need be made for biasing the grid negatively.

A further object of the present invention is to Other objects of the invention will be apparent from a consideration of the accompanying specification, claims and drawing of which the single figure is a schematic view of my control device employed as a flame detecting apparatus for a burner control system.

Referring to the drawing, the control device of the present invention is shown as located within a housing ill. This housing may be of any suitable form. If the housing is of metal, it is desirable that it be connected to ground as at ll.

Disposed within the housing is is a step-up, step-down transformer l3 having a line voltage primary it, a high voltage secondary winding t5 and low voltage secondary windings l6 and ii. The primary winding is is connected to termnals l8 and ill and by conductors Eli and 26a to line wires iii and 32 leading to any suitable source of alternating power (not shown). The low voltage secondary winding i6 is employed for energizing auxiliary apparatus to be controlled by the control device. The secondaries it and ll are employed to energize the various elements of the control device. As far as the apparatus of the present invention is concerned, the transformer it constitutes the source or power and elsewhere in the specifiation and claims, the expression source or power is used to designate the means associated with the control device for supplying power at various potentials to the control device, even though this means must in turn be connected to some primary source of power.

A relay 2t comprises a relay coil 22 and a switch blade 23 adapted to engage a fixed contact 24. Switch blade 23 is associated with an armature 25 which in turn is associated with the relay coil 22. Switch blade 23 is normally biased by gravity or other suitable means away from contact 24. Upon energization of relay coil 22, however, the armature 25 is drawn to the left to move switch blade 23 into'engagement with contact 24. i

A condenser 21 is connected by conductors 28, 29 and 30 in parallel with relay coil 22. Condenser 21 acts as a shunt condenser to eliminate any pulsations in the current flowing through relay coil 22.

The energization of relay coil 22 is controlled by an electronic discharge device 33. This device comprises a conventional envelope 34 which encloses a. cathode 35, an anode 36, and a control grid 31. The cathode 35 is of the indirectly heated type and has a heater 4| associated therewith for heating the same. The heater is connected to secondary winding ill by conductors 42 and 43. The electronic discharge device 33 is so designed that the voltage amplification is relatively uniform when a voltage is applied to the output circuit of such magnitude that the amplifier is conductive only when the potential of the grid is closer to the potential of the anode than is the potential of the cathode. In other words, the discharge device is of the type which is commonly known as a "zero cut-ofi tube."

A condenser 45 anda current limiting resistor 46 are connected between the grid ill and the cathode 35 by conductors 48, 49, 55 and B. The condenser 45 is employed to control the potential of the grid 31 under certain conditions of operation, as will be more fully explained later.

The relay '2! is illustratively shown as controlling an electrically operated gas valve 55. This gas valve is of the type which when energized permits the flow of gas and which upon deenergization interrupts such flow. The gas valve 56 controls the flow of gas through a pipe 55 to a main gas burner 55. Located adjacent to the main gas burner 56 is a pilot burner 57!, the flame of which is indicated by the reference numeral 58. The pilot burner 51! is connected to ground as indicated at 59. The pilot burner is supplied with gas by a pipe 58 which is connected to the gas supply pipe 55 at a point on the upstream side of valve 543 so that the pilot burner 5'5 is normally constantly supplied with gas and hence normally constantly burning. A flame electrode 58 is mounted so that it extends into the flame 58, when present.

The energization of valve 56 is not only controlled by relay it but also by a room. thermostat 64. This room thermostat is of conventional construction and comprises a bimetallic element iii to which is secured a contact arm 52. The contact arm is adapted to be moved into and out of engagement with a fixed contact 63. As indicated by the legends C and H, the bimetallic element ti is eflective to move the contact arm 52 to the left in the direction of contact 63 upon a temperature fall and to the right away from contact 63 upon a temperature rise. A magnet 69 is associated with contact arm 62 to impart a snap action to the contact arm and to cause a suitable difierential to exist between the temperature at which the contact arm 62 engages contact 63 and that at which it separates from contact 63.

OPERATION With proper flame The various elements are shown in the position which they occupy when the pilot burner 51 is properly ignited, but when the temperature adjacent room thermostat 64 is at or above the de-v sired value so that the thermostat 64 is satisfied. Under these conditions a conductive path will be established through the flame 58 as follows: from the lower terminal of secondary l5 through conductor 65, terminal 66, conductor 6?, flame electrode 68, burner flame 58, pilot burner 5?, ground connection 59, ground connection 6 l, conductors 10 and 50, condenser 45, and conductors 49 and H to the tap I2 of secondary i5. It will be noted that the current flow through the circuit in the direction traced is such as to impress across flame propagation. While it is possible for current to flow through the flame in both directions, it has been found that the resistance to the flow of current is considerably less when the flow of current is in a direction opposite to that of flame propagation than when it is in the same direction as flame propagation. This is due to the fact that the flow of electrons is accelerated by the ionization produced in the flame so that the flow of electrons is greater in the direction of flame propagation. Since the direction of conventional current flow is opposite to that of the flow of electrons, the conventional flow of current is greater in the direction opposite to flame propagation. Hence, the current flow through the condenser 55 in the direction just traced will be greater than that in the opposite direction so that the voltage across the condenser has a deflnite direct current component of such polarity as to tend to raise the potential of grid 31 with respect to cathode 35. The effect of this is to raise the potential of grid 37] to a value such as to cause the discharge device 33 to become conductive. Due to the fact that grid 3'! is maintained at a potential above cathode 35 within the range of values at which the discharge device is conductive, current flows through the relay winding 22 over the following circuit: from the upper terminal of secondary 15 through conductor '85, relay coil 22, conductors 30 and i6, anode 36, cathode 35, and conductors t8 and H to tap '82. Because of this flow of current through the relay 22, the relay coil 22 is maintained energized with the result that switch blade 28 is maintained in engagement with contact it.

As previously indicated, the thermostat 64 is shown in the position it assumes when the temperature adjacent bimetallic element M is at or above the desired temperature. Let it now be assumed that the temperature drops sufiiciently to cause contact blade 62 to move into engagement with contact 63. When this happens, a circuit is established to the electro-magnetic gas valve 54 as follows: from the upper terminal of secondary l6 through conductor 8!], terminal 8i, conductor 82, bimetallic element 6 I, contact blade 62, contact 63, conductor 83, terminal 84, conductor 85, switch blade 2-3, contact 24, conductor 88,

terminal 81, conductor 88, valve 54, conductor 89, terminal 90, and conductor 9! back to the lower terminal of secondary l6. As a result of the establishment of this circuit, the operating mechanism of valve 54 is energized with the result that the valve is moved to open position so as to permit gas to flow to the main burner 56. The gas flowing to the main burner 56 is ignited by the pilot burner 58.

Normally, the burner 56 will remain ignited until the thermostat 64 becomes satisfied. When this happens, the contact arm 62 separates from. contact 63, thus interrupting the energizing circuit to the valve 54 and causing valve 54 to close.

When pilot burner is extinguished For any of various reasons, the pilot burner 5'! may become extinguished. For example, the flow of gas may be momentarily terminated with the result that the pilot flame becomes extinguished and it is necessary to relight the same. The control device of the present invention is designed to detect almost instantly any termination of the pilot burner flame. As soon as the pilot burner flame is interrupted, the circuit which was traced through condenser 45 is interrupted, it being recalled that this circuit included the assasoa flame II. The charge on condenser 45 is now dissipated and the grid 31 is again at the same potential as the cathode 35. As previously pointed out, the voltage applied to the anodecathode circuit of tube 34 is such that when th grid is at the same potential as the cathode, the amplifier is non-conductive. As a result, as soon as the flame is extinguished so that condenser 45 is no longer charged and the grid 31 assumes the cathode potential, the flow 01. current to amplifier 33 will be terminated. The result of this is that relay coil 22 is deenergized so that switch blade 23 separates from contact 24. This will interrupt the circuit through the gas valve 54 and regardless of whether the thermostat 54 is calling for heat or not, the gas valve 54 will assume closed position in which further flow of gas to the main burner 55 is prevented.

It will thus be seen that whenever the flame 53 is extinguished for any reason whatsoever, the relay coil 22 is quickly deenergized to cause separation of switch blade 23 from contact 24.

When flame gap is bridged by a high resistance leak The flame electrode 68 is normally at a substantially difierent potential than ground. This is necessary in order for an appreciable current flow to take place through the flame gap. Obviously, when such a high potential difference is maintained between the flame gap and ground, a very slight deiect'in the insulation of the flame gap may result in a condition simulating the effect 01' a flame, as far as resistance is concerned. The present control device is designed to distinguish such a condition from a normal flame condition, even if the resistance drop through the resistance leak is identical to the normal resistance of a flame.

It will be recalled that the charging of condenser 45 is dependent upon the current flow through flame 58 being greater in one direction than in the other. This rectifying effect of the flame is not present in the case of a resistance leak, the resistance through such a leak being the same in both directions. As a result, the current flow through this resistance leak and through condenser 45 is identically the same in both directions so that the condenser 45 has merely an alternating potential impressed on it and, as long as the resistance value of the leak is high, this potential is small. Since grid 36 is connected to the right-hand terminal of condenser 45, as previously noted, the grid 31 is connected through the lower terminal of secondary [5 through the flame gap as follows: from grid 31 through conductor 5|, resistor 45, conductor 10, ground connections II and 59, pilot burner 51, burner flame 58, electrodes 58, conductor 61, terminal 66 and conductor 55.

As soon as the current flow through condenser 45 is the same in both directions, the grid has impressed upon it only the alternating potential due to the connection just traced. The potential at the lower end of secondary l5, as far as cathode 35 is concerned, is 180 degrees out of phase with respect to the potential at the upper end of secondary l5 to which the anode 35 is connected. Thus, the tendency of the connection just traced is to impress upon grid 31 an alternating potential which is 180 degrees out of phase wth respect to the potential applied between the anode and cathode. As previously noted, the magnitude of this potential is relatively small as long as the resistance of the flame gap is large. Nevertheless, the effect of this alternating potential, no

matter how slight, is to cause the grid to assume When short circuit exists across flame cap If there is a short circuit across the flame gap, such as would occur it the flame electrode 3! accidentally engaged the pilot burner, the apparatus again will respond as though a flame is not present. As previously indicated, the current flow through the gap when the flame gap is bridged by any non-rectifying impedance is the same in both directions so that only an alternating current potential is impressed across the condenser. In describing the operation 01 the high resistance leak, it was pointed out that the grid is connected through the flame gap to the lower end of secondary l5. It was pointed out that the resistance of this connection is relatively high as long as the resistance of the flame gap is high. Obviously, in the event of a short circuit condition, the resistance or this path drops appreciably so that the grid potential becomes even closer to the potential at the lower end of the cathode ii. The result of this is to drive the grid potential even more negative with respect to the cathode. The protective resistance 46 is interposed to reduce this potential drop to some extent. If it were not for the protective resistance 45, the grid 31 would assume a value very highly negative with respect to cathode 35 and the resulting potential diflerence might well cause injury to the elements of the tube 33. In

any event, it will be obvious that the effect of a short circuit is to render the amplifier 33 even more non-conductive so as to insure deenergizetion of the relay 22.

CONCLUSIQN tween the grid 31 and only a single point on the source of power. This connection is through resistor 46 and through the flame gap to the lower terminal of secondary 15. tion, moreover, is one which would. normally tend to cause the grid 31 to assume a potential in which no current flow could take place through the amplifier 33. Because of the presence of condenser 45, however, the effect of this connection is changed when the flame gap is bridged by a rectifying impedance. Under these conditions, the presence of this connection actually causes the grid to assume a potential above that of thecathode so that the tube becomes conductive. Thus, because of the condenser 45, a connection which would normally tend to bias the grid negatively actually results in the potential of the grid being increased. When there is no flame present at all, this connection is entirely interrupted so that the grid assumes the same potential as the cathode to cause the amplifier to become non-conductive.

While the control device has been described in connection with an application in which the gap between terminal 66 and ground II was bridged by a flame gap, it is to be understood that as far as certain aspects of the invention are concerned, the gap could be bridged by any other impedance which under certain conditions is rectifying. For example, in certain cases, it may be desirable to connect a photoelectric cell This connecalliance between terminal 86 and the ground connection H. Such a photoelectric cell will conduct current in only one direction and only when subjected to a source of light. It will be obvious that where such a photoelectric cell is connected, the apparatus will iunction in the manner intended only when the photoelectric cell is exposed to the source of light. If a short circuit condition is established around the photoelectric cell, the apparatus will function in the same manner as described in connection with the disclosed embodiment when a short circuit condition is established across the flame sap. Similarly when the photoelectric cell is not exposed to the source of light, a substantially open circuit condition exists between teal 8t and ground it so that the apparatus functions in the same manner as described when no flame exists in connection with the illustrative embodiment oi the apparatus.

It is accordingly to be understood that in the appended claims, when reference is made to means exposed to a burner flame, this language is to be construed broadly enough to be readable upon both a dame electrode disposed in a burner flame and a photoelectric cell exposed to the light from such a burner flame.

In general, while I have disclosed a specific embodiment of my invention, it is to be understood that this is for purposes of illustration only and that my invention is to be limited only by the scope of the appended claims.

I claim:

1. In combination; an electrical device; a space discharge amplifier having an anode, a cathode, and a control element, said amplifier controlling the energization of said device; connections irom a source of alternating voltage to said anode and cathode for applying between said anode and cathode an alternating voltage of such magnitude that said amplifier is efiectively conductive only if the control electrode potential i closer to the anode potential than is that of the cathode; a. connection between said control electrode and a point on said voltage source whose voltage phase with respect to the cathode is opposite to that of the point to which the anode is connected so as to tend to maintain said control electrode at a potential at which the amplifier is eflectlvely non-conducting, said connection including a gap adapted to be bridged by a rectifying impedance; and means associated with said connection and efi'ective when said gap is bridged'by a rectifying impedance to maintain said control electrode at a potential at which the amplifier is effectively conductive.

2. In combination; an electrical device; a. space discharge amplifier having an anode, a cathode, and a control element, said amplifier controlling the energization of said device; connections from a source of alternating voltage to said anode and cathode for applying between said anode and cathode an alternating voltage of such magnitude that said amplifier is efiectively conductive only if the control element potential is closer to the anode potential than is that of the cathode; a connection between said control electrode and a point on said voltage source whose voltage phase with respect to the cathode is opposite to that of the point to which the anode is connected so as to tend to maintain said control element at a potential at which the amplifier is eilectively non-conducting, said connection including a gap adapted to be bridged by a rectifying impedance; and a condenser cathode and effective when said gap is bridged by a rectifying impedance to assume a charge such as to maintain said control element at a potential at which the amplifier is effectively conductive.

3. In combination; an electrical device a space discharge amplifier having an anode, a cathode, and a control element, said amplifier controlling the energization of said device; connections from a source of alternating voltage to said anode and cathode for applying between said anode and cathode an alternating voltage of such magnitude that said amplifier is effectively conductive only if the control element potential is closer to the anode potential than is that of the cathode; a connection between said control electrode and a point on said voltage source whose voltage phase with. respect to the cathode is opposite to that of the point to which the anode is connected so as to tend to maintain said control element at a potential at which the amplifier is efiectively non-conducting, said point being the only point on said voltage source independent of the cathode connection to which said control element is connected; said connection including a gap adapted to be bridged by a rectifying impedance, and means associated with said connection and efiective when said gap is bridged by a rectifying impedance to maintain said control element at a potential at which the amplifier is efiectively conductive.

4. In combination; an electrical device; a space discharge amplifier having an anode, a cathode, and a single control element, said amplifier controlling the energization of said device; connections from a source of alternating voltage to said anode and cathode for applying between said anode and cathode an alternating voltage of such magnitude that said amplifier is effectively conductive only if the control element potential is closer to the anode potential than is that of the cathode; a connection between said control element and a point on said voltage source whose voltage phase with respect to the cathode is 0pposite to that of the point to which the anode is connected so as to tend to maintain said control element at a potential difiering from the-anode potential by more than the cathode potential so that the amplifier is effectively non-conducting, said connection including a. gap adapted to be bridged by a rectifying impedance; and a condenser connected between said control element and cathode and eiiectlve when said gap is bridged by a rectifying impedance to assume a charge such as to maintain said control element at a potential at which the amplifier is effectively conductive, said condenser being connected to said source of voltage only by the aforesaid connections of said cathode and control element to the source of power so that when said gap is open, no charge exists on said condenser.

5. In combination; an electrical device; a space discharge amplifier having an anode, a. cathode, and a single control element, said amplifier controlling the energization of said device; connections from a source of alternating voltage to said anode and cathode for applying between said anode and cathode an alternating voltage of such magnitude that said amplifier is effectively conductive only if the control element potential is closer to the anode potential than is that of the cathode; a condenser connected between the cathode and said control element; and a charging circuit for said condenser including a portion oi said source of alternating voltage and a gap adapted to be bridged by an impedance having rectifying characteristics; said condenser being eflective when charged by said circuit upon said gap being bridged by a rectifying impedance to cause the potential 01 the control element to assume a value between that of the cathode and that of the anode; said charging circuit constituting the only means for connecting said condenser to said source oi! power so that when said gap is open, no charge exists on said condenser.

6. In a flame detecting apparatus; an electrically operated device whose energization is to be controlled in accordance with the presence or absence of a flame; a space discharge amplifier having an anode, a cathode, and a single control element, said amplifier contro1- ling the energization of said control device;

connections from a source of alternating volttage to said anode and cathode for applying between said anode and cathode an alternating voltage of such magnitude that said amplifier is effective to cause eii'ective energization of said device only if the control element potential is closer to the anode potential than is that of the cathode; a condenser connected between the cathode and said control element; meansadapted to be exposed to a flame and acting as a rectifying impedance when so exposed and a charging circuit for said condenser including a portion of said source of alternating voltage and said last named means; said condenser being effective when charged by said circuit upon said means being exposed to a flame to cause the potential of the control element to assume a value between that of the cathode and that of the anode; said charging circuit constituting the only means for connecting said condenser to said source of power so that when said gap is open or bridged by a non-rectifying impedance, no charge exists on said condenser.

7. In a flame detecting apparatus; an electrically operated device whose energization is to be controlled in accordance with the presence or absence of a flame; a space discharge amplifier having an anode, a cathode, and a control element, said amplifier controlling the energization of said device; connections from a source of alternating voltage to said anode and cathode for applying between said anode and cathode an alterhating voltage of such magnitude that said amplifier is efiective to cause effective energization of said device only if the control element potential is closer to the anode potential than is that of the cathode; a condenser connected between the cathode and said control element; and a charging circuit for said condenser including a portion of said source of alternating voltage and a gap adapted to be bridged by a flame; said circuit being effective when said gap is bridged by a fiame to charge said condenser with a direct current voltage by reason of the rectifying properties of the flame; said condenser being effective when charged by said circuit to cause the potential oi the control element to assume a value between that of the cathode and that of the anode; said charging circuit constituting the only means for connecting said condenser to said source of power so that when said gap is open or bridged by a non-rectifying impedance, no charge exists on said condenser.

8. In combination; an electrical device; a source of alternating voltage; a space discharge amplifier having an anode, a cathode, and a contro1 element; connections between said space discharge amplifier and said source of power consisting entirely oi conductors connecting said electrical device between said anode and a first point on said source of voltage, conductors connecting said cathode to a second point of said source of voltage, and a connection between said control element and a third point on said source of voltage whose voltage phase with respect to said second point is opposite to that of the first point, said connection having a gap therein which is adapted to be bridged by a rectifying impedance; and a condenser connected between said control electrode and said cathode in parallel with that portion of said source of voltage between said second and third points; said condenser being of a capacity such that when said gap is bridged by an impedance which is rectifying in the proper direction, the resulting charge on said condenser is efiective to cause the control element to assume a potential at which said amplifier is conductive.

9. In flame detecting apparatus; an electrically operated device whose energization is to be controlled in accordance with the presence orabsence of a flame; a space discharge amplifier having an anode, a cathode, and a control element; a source of alternating power; circuit connections for controlling said electrical device including said anode, said cathode, and said source of power and operative to cause said electrical device to be energized when the potential of said control element is within a first of two ranges of values and effectively deenergized when said control element potential is within a second of said two ranges of values; a condenser connected between said control element and said cathode; means adapted to be exposed to a flame and acting as a rectii'ying impedance when so exposed; and a connection including said last named means and extending between the source of power and the terminal of said condenser connected to said control element, said connection being the only connection between said terminal and said source of power and being effective when said means is exposed to a flame to cause said condenser to be charged with a voltage of a first polarity such as to cause said control element to be maintained at a potential within one of said ranges of values; said connection being to a point on said source of power whose voltage phase with respect to the cathode is such that when said fiame responsive means is bridged by a symmetrically conductive impedance, the connection tends to cause the voltage across said condenser to have a polarity opposite to said first polarity during the half cycles in which the anode is positive with respect to the cathode.

10. A flame detector comprising an electrical device; a space discharge amplifier having an anode, a cathode, and a control element; a source of alternating power; circuit connections for controlling said electrical device including said anode, said cathode, and said source of power and operative to cause said electrical device to be energized when the potential of said control element is within a first of two ranges of values and to be efiectively deenergized when the potential of said control electrode is within a second of said two ranges of values; a condenser connected between said control element and said cathode; and a connection including a flame gap and extending between the source of power and the terminal of said condenser connected to said cont-r01 element, said connection being the only connection between said terminal and said source of power and being eflective when said flame gap is bridged by a flame to cause said condenser to be charged with a voltage of a first polarity such as to cause said control element to be maintained at a potential within one of said ranges of values; said connection being to a point on said source of power whose voltage phase with respect to the cathode is such that when said flame gap is bridged by a symmetrically conductive impedance, the connection tends to cause the voltage across said condenser to have a polarity opposite to said first polarity during the half cycles in which the anode is positive with respect to the cathode.

11. In flame detecting apparatus; an electrically operated device whose energization is to be controlled in accordance with the presence or absence of a flame; a space discharge amplifier having an anode, a cathode, and a control element; a source of alternating power; circuit connections for controlling said electrical device including said anode, said cathode, and said source of power; a connection between said control electrode and a point on said source of power whose voltage phase with respect to the cathode is opposite to that of the anode so that the connection tends to maintain said control element at an alternating potential at which said amplifier is inoperative to efiectively energize said device, said point being the only point on the power source independent of the cathode connection to which said control element is connected; said last named connection including means adapted to be exposed to a flame and acting as a rectifying impedance when so exposed; and means associated with said last named connection and efiective when said means adapted to be exposed to a flame is so exposed to maintain said control element at a different potential such that said amplifier is operative to efl'ectively energize said device.

12. In combination; an electrical device; a space discharge amplifier having an anode, a cathode, and a control grid; a transformer having a primary winding and a secondary winding; connections between spaced points on said secondary winding and said anode and cathode; said connections including said electrical device and constituting an output circuit for said amplifier; a condenser connected between said control grid and said cathode independently of said transformer; a resistor; and means for charging said condenser comprising a series circuit including said condenser, said resistor, and a gap adapted 12 to be bridged by a rectifying impedance having its direction of greatest conductivity such that said condenser is so charged as to increase the potential of said grid in a positive direction with respect to said cathode.

13. In combination; an electrical device; space discharge amplifier having an anode, a cathode, and a control grid; a transformer havinga primary winding and a secondary winding; connections between spaced points on said secondary winding and said anode and cathode; said connections including said electrical'device and constituting an output circuit for said amplifier; said amplifier being of such character and being so connected to said secondary winding that said lmplifier is eiiective to energize said device only when said control grid is positive with respect to said cathode; a condenser connected between said control grid and said cathode independentlyof said transformer; a resistor; and means for charging said condenser comprising a series circuit including said condenser, said resistor, and a gap adapted to be bridged by a rectifying impedance having its direction of greatest conductivity such that said condenser is so charged as to increase the potential of said grid in a positive direction with respect to said cathode.

JOHN M. WILSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,947,189 Cockrell Feb. 13, 1934 2,003,992 Cockrell June 4, 1935 2,049,355 Cockrell July 28, 1936 2,120,053 Fehrenbach June 7, 1938 2,136,256 Sweet Nov. 8, 1938 2,243,071 Crago l May 27, 1941 2,299,501 Schneider Oct. 20, 1942 2,343,001 Cohen Feb. 29,1944 2,352,240 Wolfner, 2d June 27, 1944 2,360,532 Yates Oct. 17, 1944 2,379,871 Beam et a1. July 10, 1945 2,379,872 Gille July 10, 1945 2,379,873 Lange July 10, 1945 2,403,117 Shafler June 11, 1946 2,410,524 Richardson et al. Nov. 5, 1946 2,416,781 Thomson Mar. 4, 1947 

